INTRODUCTION: The effect of TSH on the development and progression of thyroid cancer is not clear. This study was performed to investigate the effect of TSH on the tumor size and invasiveness in well-differentiated thyroid cancer. METHODS: Three hundred and thirty-one patients undergoing thyroidectomy in general surgery department of our hospital between 2015 and 2017 were evaluated. Laboratory findings, demographic characteristics and the pathology reports of patients were retrospectively evaluated. Patients with normal preoperative serum TSH levels (0.4-4.0 mU/L) and below the lower limit of the normal range (0.4 mU/L) were classified as Normal-TSH Group (NTG) and Low-TSH Group (LTG); respectively. Tumor multicentricity, tumor capsular invasion, thyroid carcinoma with capsular invasion and lymphovascular invasion were investigated. Chi-square test, Students t-test, and MannWhitney U tests were used for statistical analysis.RESULTS: Differentiated thyroid cancer was determined in a total of 73 patients including 63 in NTG and 10 in LTG. Mean tumor diameter was determined to be 15.52±14.01 mm and 14.00±15.47 mm in NTG and in LTG; respectively. There was no statistically significant difference between the two groups with respect to tumor size (p=0.450). Although NTG had a higher rate of tumor multicentricity, tumor capsular invasion, thyroid carcinoma with capsular invasion and lymphovascular invasion, there was no statistically significant difference between the two groups.DISCUSSION AND CONCLUSION: In our study, no relationship was determined between preoperative serum TSH levels and tumor size and invasiveness. Prospective, randomized controlled studies are needed on this subject.